A base station antenna for mobile communication is designed by means of a space diversity scheme or a polarization diversity scheme so as to reduce a fading phenomenon. A space diversity scheme means to install a transmitting antenna and a receiving antenna while being spaced a predetermined distance from each other, and has a large limit in space and a disadvantage in cost. Accordingly, a mobile communication system has typically used a dual-band dual-polarized antenna to which a polarized diversity scheme is applied.
A dual-band dual-polarized antenna is used to transmit (or receive) two linear polarizations which are arranged rectangular to each other, e.g. which can be vertically and horizontally arranged, respectively. However, it is very important to operate the dual-band dual-polarized antenna so as to allow these polarizations to be arranged at +45 degrees and at −45 degrees respective to a vertical direction (or a horizontal direction). Generally, a dual-band dual-polarized antenna is operated in two frequency bands which are sufficiently spaced apart from each other. An embodiment of such a dual-band dual-polarized antenna is disclosed in the U.S. Pat. No. 6,333,720 (title: dual-polarized multi-range antenna) filed by Kathrein-Werke.
FIG. 1 is a perspective view illustrating an embodiment of an array of a conventional dual-band dual-polarized antenna, which is the same as what is disclosed in the U.S. Pat. No. 6,333,720. With reference to FIG. 1, a conventional dual-band dual-polarized antenna includes the first radiation device module 1 for the first frequency band (a lower frequency band, hereinafter, referred to as a low frequency band) and the second radiation device module 3 for the second frequency band (a higher frequency band, hereinafter, referred to as a high frequency band).
Two radiation device modules 1 and 3 are arranged on a conductive reflection plate 5 having a substantially square shape. A feeding network can be positioned at a rear surface of the conductive reflection plate 5 so that each of the first and second radiation device modules 1 and 3 is electrically connected. The first radiation device module includes a plurality of dipoles 1a generally arranged to form an square shape, and the dipoles 1a are mechanically supported by a reflection plate 5 or a plate positioned at the rear place thereof by means of what is called a balancer 7, and also make electric contact therewith. At this time, the reflection plate 5 has side walls 6, which extend from a corresponding plane while having a proper height, at both edges thereof so as to improve a radiation characteristic.
A dipole device of the first radiation device module 1 has a set length so as to allow corresponding electromagnetic waves to be transmitted and received through the corresponding dipole device. Therefore, in the dual-polarized antenna, dipole devices are exactly arranged while meeting at right angles. Typically, each of the dipole devices 1a is arranged at +45 and −45 degrees respective to the vertical direction (or respective to a horizontal direction) so that they form an antenna which is briefly named an X-polarized antenna.
The second radiation device module 3 can be positioned within the first radiation device module 1 having a square shape formed by dipoles or at the exterior thereof. Such a second radiation device module 3 has dipoles which are arranged not to form a square shape but to form a cross-shape. Similarly, two dipoles 3a positioned at a right angle to each other are supported by the reflection plate 5 by means of a corresponding balance net, and are fed with power through it.
The first and second radiation device modules 1 and 3 are exactly arranged at proper positions on the reflection plate 5. At this time, the second radiation device module is arranged within the first radiation device module 1. Also, as shown in FIG. 1, two antenna apparatuses formed by such first and second radiation device modules 1 and 3 can be installed at the reflection plate 5 in a vertical direction, and the second separated radiation device module 3⊚ of the second frequency band can be installed in the space between the two antenna apparatuses, thereby obtaining high vertical benefit through such an arrangement scheme.
However, in the structure of the antenna as shown in FIG. 1, it is difficult to install a side wall so as to adjust the width of a beam. Particularly, a mobile communication station is divided into three sectors, and the width of the beam of a sector antenna is adjusted at 65 degrees or 90 degrees. So as to secure the width of the beam at 65 degrees, it is adjusted by the selection of a radiation device, the distance between side walls, and the height of the side walls. The structure of the antenna as shown in FIG. 1, a square-shaped radiation device of a low frequency band is arranged to form a rhombic shape respective to a vertical direction, so that if a side wall is escaped from the radiation device in a high degree, or if a side wall is adjusted to the size of radiation device, the size of the side wall becomes larger. As the side wall is near the radiation device, it is easy to adjust the width of the beam thereof. Therefore, it is difficult to simultaneously adjust the low frequency band and high frequency band to the width of the beam at 65 degrees.
Accordingly, in the conventional dual-band dual polarized antenna, it is difficult to adjust the width of the beam so that the characteristic of the antenna, e.g. separation degree and cross deviation, are deteriorated so as to firstly adjust the width of the beam.